How Barrier Islands Work

The structure of a typical barrier island consists of the following zones, from the ocean side toward the sound:

Beach - consists of sand deposited by the actions of waves

Dunes - formed from sand carried and deposited by winds. Dunes are stabilized naturally by plants (sea oats, bitter pancum) and artificially by fences. The primary dune faces the ocean and may be followed by secondary and tertiary dunes inland.

Barrier flat - (also called backdune, overwash or mud flat) formed by sediments that get pushed through the dune system by storms, such as hurricanes. Grasses grow and stabilize these areas.

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Storms push sediments through to form the overwash

Photos courtesy USGS

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Storms push sediments through to form the overwash

Photos courtesy USGS

Salt marsh - a low-lying area on the sound-side of a barrier island. Salt marshes are generally divided into high and low marsh areas. High marsh areas get flooded twice each month with the spring tides, while low marsh areas get flooded twice daily with the high tides. Cord grasses stabilize the salt marsh area, which are one of the most ecologically productive areas (amount of vegetation per acre) on Earth. In fact, the salt marsh ecosystems of the islands and the coast help to purify runoffs from mainland streams and rivers.

Each of these zones has distinct animal and plant life, which we will discuss in the next section.